1. Field of the Invention
The present invention relates to a high pressure fuel injection tube used widely in, for example, a supply path or the like of fuel in a diesel internal combustion engine and having a connection head portion by a thick-walled steel tube of a comparatively slender diameter having a tube diameter of 4 mm through 20 mm and a wall thickness of about 1 mm through 8 mm and a method of forming the same.
2. Description of Related Art
There is known a high pressure fuel injection tube of this kind of a background art arranged to be used in a supply path of fuel of a diesel internal combustion engine or the like having a connection head portion 22 formed by a seat face 23 in a spherical shape, a ring-like flange portion 25 formed to be arranged to be spaced apart from the seat face 23 in an axis core direction at an interval therebetween and a conical portion 24 formed conversingly in a direction of the seat face 23 at an interval region reaching the ring-like flange portion 25 continuously from the seat face 23 at a connection end portion of a thick-walled steel tube 21 having a comparatively slender diameter as shown by, for example, FIG. 7 (refer to JP-A-8-75075).
It is general that the connection head portion 22 of this kind is constituted to produce a deep, sharp, and large ring-like pocket 26 at an inner peripheral face of the head portion by spreading a peripheral wall to an outer side in accordance with buckling in relation to forming by the buckling by being pressed in the axis core direction by a punching member from outside and is subjected to use under such a state.
However, the high pressure fuel injection tube having such a connection head portion poses a problem that cavitation erosion is brought about at the pocket portion owing to a high pressure fluid in being arranged to be used by the deep, sharp and large thin-like pocket 26 formed at an inner peripheral face of the connection head portion 22 and a problem that there is the threat of possibility of constituting an onset of fatigue rupture by the deepest portion of the pocket portion.
In order to resolve such a problem, the applicant has previously proposed a high pressure fuel injection tube having a connection head portion capable of eliminating a concern by cavitation erosion and the threat of fatigue rupture on an inner side of the head portion by forming a ring-like pocket produced at inside of the head portion with by working the connection head portion into a contour shape of a section as shallow and smooth as possible (refer to JP-A-2003-336560).
According to the high pressure fuel injection tube, as shown by FIG. 8, in the high pressure fuel injection tube having a connection head portion 32 constituted by a seat face 33 in a spherical shape, a ring-like flange portion 35 formed to be arranged to be spaced apart from the seat face in an axis core direction at an interval therebetween and a conical portion 34 formed conversingly in a direction of the seat face at an interval region reaching the ring-like flange portion 35 continuously from the seat face 33 at a connection end portion of a thick-walled steel tube 31 having a comparatively slender diameter, a ring-like recess groove (pocket) 36 formed at an inner peripheral face of the connection head portion in accordance with forming the connection head portion 32 is formed into a contour shape having a shallow smooth section by providing a bent recess groove 34-1 in a ring-like shape having a shallow depth at a portion of the conical portion 34.
That is, according to the high pressure fuel injection tube, a portion of conical face of the connection head portion is pinched by exerting more pressure, by operation of pressing a projected bent face corresponding to the contour shape of the bent recess groove 34-1 previously provided on the side of a forming punch, the bent recess groove 34-1 in the ring-like shape having the shallow depth at a portion of the conical portion 34 of the connection head is formed, thereby the depth of the ring-like recess groove at the inner peripheral face of the connection head produced by buckling can be made to be shallow and smooth, cavitation erosion by a hydraulic pressure at the ring-like recess groove at the inner peripheral face of the connection head portion is prevented from being brought about and the threat of a possibility of constituting an onset of fatigue rupture by stress concentration is reduced.
Although according to the high pressure fuel injection tube previously proposed by the applicant, by providing the ring-like recess groove having the shallow depth at a portion of the conical portion of the connection head portion, the depth of the ring-like recess groove (pocket) on the inner side of the connection head portion produced by buckling is made to be shallow and smooth, thereby, there is achieved an excellent effect of eliminating the concern of cavitation erosion by the hydraulic pressure at inside of the connection head portion and capable of reducing the threat of the possibility of constituting the onset of the fatigue rupture, since there is hardly present a compressive residual stress extremely effective in increasing a strength against the fatigue rupture at the ring-like recess groove portion on the inner side of the connection head portion, there poses a problem that the strength against the fatigue rupture at the ring-like recess groove portion is not sufficiently achieved.
That is, by only making the ring-like recess groove (pocket) on the inner side of the connection head portion produced by buckling shallow and smooth by providing the ring-like recess groove having the shallow depth at a portion of the conical portion of the connection head portion, since the depth of the ring-like recess groove provided at an outer peripheral face of the conical portion of the connection head portion is shallow, the connection head portion after buckling is brought into a tensile residual stress state, and there is brought about a state in which the compressive residual stress extremely effective in increasing the strength against the fatigue rupture is hardly present at the ring-like recess groove (pocket) on the inner side of the connection head portion.